This application is directed to a process for disposition of the aqueous sulfate by-product from the manufacture of 2-hydroxy-4-(methylthio)butyric acid and more particularly to a process for regenerating sulfuric acid from the sulfate by-product. The sulfuric acid may be recycled the 2-hydroxy-4-(methylthio)butyric acid (hereinafter "HMBA") manufacturing process in an essentially closed circuit operation.
The invention is further directed to a process for disposal of HMBA where, for example, that product may be contaminated with adulterating components, or otherwise unsuitable for sale, by use of the HMBA as a sulfur bearing and caloric feed to a spent acid regeneration process.
Ruest U.S. Pat. No. 4,524,077 describes a process for the preparation of a concentrated aqueous solution of HMBA. In this process, the product is produced by sulfuric acid hydrolysis of 2-hydroxy-4-(methylthio)butronitrile (hereinafter "HMBN") and the resulting hydrolyate is extracted with a substantially water-immiscible solvent for product recovery. Steam distillation of the extract yields a bottom product comprising the desired concentrated aqueous solution of HMBA. The raffinate from the extract is stripped for recovery of solvent, thereby producing a bottom fraction which comprises a solution of ammonium bisulfate. Depending on hydrolysis conditions, it may also contain some portion of either ammonium sulfate or sulfuric acid, as well as small portions of HMBA. Minor quantities of organic sulfide impurities such as methyl mercaptan, dimethyl sulfide, and dimethyl disulfide are also purged from the process.
Various alternatives exist for disposal of the sulfate by-product solution from the Ruest process. Disposition of this material as an ammonium salt by-product or waste stream results in a substantial consumption of sulfuric acid in the HMBA manufacturing process. Environmental regulations limit the options for waste disposal and generally increase the cost thereof. Although a certain amount of ammonium sulfate can be used as a fertilizer, the modest nitrogen content limits the attractiveness of the by-product salt for this purpose. Moreover, recovery of the sulfate by-product in the form of fertilizer grade ammonium sulfate requires neutralization and purification operations that involve substantial capital and operating expense.
Hernandez et al. U.S. Pat. No. 4,912,257 describes an alternative process for the manufacture of concentrated aqueous solutions of HMBA. Hernandez et al. produces HMBA by essentially the same hydrolysis reaction as Ruest, i.e., by reaction of HMBN with sulfuric acid. Instead of recovering the HMBA product from the hydrolyzate by extraction, Hernandez et al. use the known tendency of the hydrolyzate to separate into aqueous and organic phases as a means for producing an initial separation between HMBA and by-product salt. Hernandez et al. neutralize the hydrolyzate before phase separation so that the aqueous phase withdrawn from the separator contains a high percentage of ammonium sulfate and a relatively small amount of ammonium bisulfate. After phase separation, the aqueous phase is concentrated by evaporation to precipitate a portion of the ammonium sulfate, which is separated by filtration and removed from the process. The filtrate is recycled to the neutralization step.
Although the ammonium sulfate precipitate produced by evaporative concentration of the aqueous phase from the Hernandez et al. phase separation is said to be relatively pure, the market for ammonium sulfate is limited.
Conventionally, therefore, the ammonium bisulfate and ammonium sulfate by-products of HMBA preparation have been largely wasted.
Various processes have long been known for the regeneration of sulfuric acid from certain sulfate bearing waste materials. Isbell et al. U.S. Pat. No. 3,549,320 describes a process for regenerating sulfuric acid from sulfate wastes that are produced in the manufacture of methylmethacrylate ("MMA"). In the process of producing MMA, acetone cyanohydrin is reacted with fuming sulfuric acid to produce methacrylamide sulfate. The methacrylamide sulfate is then reacted with methanol and water to produce MMA and by-product ammonium bisulfate. Other compounds which may be present in the resulting mixture include sulfuric acid, ammonium sulfate, C.sub.3 H.sub.4 O(NH.sub.4 SO.sub.3).sub.3, and organics having the empirical formula C.sub.3 H.sub.10 O.sub.3. In the Isbell et al. process, this solution is burned in a flame produced by burning a fossil fuel. The products of combustion include carbon dioxide, oxygen, water, sulfur dioxide, nitrogen, oxides of nitrogen, and a small fraction of sulfur trioxide. The effluent gas is scrubbed, dried, heated to 300.degree.-500.degree. C., and passed over a conversion catalyst in the presence of added amounts of oxygen. The SO.sub.2 is catalytically oxidized to SO.sub.3, and the SO.sub.3 is removed from the gas by absorption in oleum. Further amounts of oleum thereby produced are recycled and used for reaction with further quantities of acetone cyanohydrin in the formation of methacrylamide sulfate.
Gelblum et al. U.S. Pat. No. 4,490,347 describes an improvement in the process of Isbell. Gelblum uses an oxygen-enriched flame in the combustion furnace in order too reduce air requirements, thereby allowing the inert content of the combustion gas to be reduced, and the SO.sub.2 content to be increased. Preheating of the air and sulfate solution fed to the furnace further reduces the fuel consumption and the inert fraction, thus further increasing the SO.sub.2 gas strength. Gelblum also mentions the alternative of recycling to the furnace a fraction of the stack gas, i.e., the gas exiting the absorber which contains residual portions of sulfur dioxide. The process of Gelblum, like that of Isbell, is specifically directed to the regeneration of sulfuric acid from the sulfate waste of an MMA process.
In addition to the need for disposal of the sulfate waste from a conventional process for the manufacture of HMBA, an occasional need may arise for the disposal of fractions of HMBA itself, for example, HMBA which does not meet published product specifications. HMBA is typically used as an essential amino acid source in the preparation of animal feeds. If an animal feed processor inadvertently mixes the HMBA with other materials not intended for a feed mix, or mixes in the wrong proportions, the HMBA mixture may become unusable, and a need may arise for an economical and environmentally safe method of disposing of the HMBA mixture.